1. Field of Invention
The present invention relates to articles for treating combustion exhaust gas. More particularly, the present invention relates to particulate filters coated with a catalyst for reducing soot and other undesirable components from lean burn combustion exhaust gas.
2. Description of Related Art
The largest portions of most combustion exhaust gases contain relatively benign nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2); but the exhaust gas also contains in relatively small part noxious and/or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (mostly soot). To mitigate the environmental impact of exhaust gas released into the atmosphere, it is desirable to eliminate or reduce the amount of these undesirable components, preferably by a process that does not generate other noxious or toxic substances.
Certain lean-burn engines, such as diesel engines, tend to produce an exhaust gas with a considerable amount of soot and other particulate matter. Soot emissions can be remedied by passing the soot-containing exhaust gas through a diesel particulate filter (DFP), such as a wall-flow filter.
To reduce the amount of space required for an exhaust system, it is often desirable to design individual exhaust components to perform more than one function. For example, applying a catalyst coating to a wall-flow filter substrate serves to reduce the overall size of an exhaust treatment system by allowing one substrate to serve two functions, namely remove soot and serve as a substrate for a heterogeneous catalyst. However, coating the filter with an operable amount of catalyst can undesirably increase the backpressure across the filter which, in turn, reduces engine performance and fuel economy. This is particularly true for high performance catalyst washcoats, such as selective catalytic reduction (SCR) catalyst comprising transition metal promoted zeolites.
Many attempts have been made to prepare washcoated DPFs having a suitably high amount of catalyst and suitably low backpressure. However, there remains a need for DFPs which produces a relatively low backpressure when coated with an effective amount of an SCR catalyst.